Cellular telephone and mobile telephone systems are becoming more commonplace. In third generation mobile systems that are currently being developed, the mobile system must support a mobile speed up to 500 Km/Hr, which incurs a large Doppler frequency shift. Many of these mobile systems use rake receivers for enhanced multipath discrimination and other advantages. These receivers are used with spread spectrum communication signals, such as a code division multiple access (CDMA) communication system, where the rake receiver performs continuous, detailed measurements of multipath characteristics to combat selective fading. This can be accomplished in some rake devices by detecting a signal from each path individually, using correlation methods and algebraically combining echo signals into a single detected signal. Most rake receivers use rake “fingers” or sections, which combine signals received from the various paths. The rake fingers can be analogized to matched filters, where path gains of each “finger” work similar to matched filter taps. Examples of various rake receivers and rake “finger” structures are disclosed in U.S. Pat. Nos. 5,659,573; 5,910,950; 6,085,104; and 6,163,563.
As noted before, the mobile systems support high speeds are subject to Doppler frequency shift. One conventional method for solving and eliminating Doppler frequency shift within spread spectrum receivers and typically rake receivers, is the use of channel estimation to estimate Doppler frequency shift. This type of system, however, requires a complex filter structure and an optimum filter, such as Wiener filter, that is not realizable in many circuits.